Interpretive Summary: When lettuce is cut and then washed, juice from the lettuce react with the commonly used sanitizer, chlorine, and thus deplete chlorine’s ability to kill harmful bacteria. The surviving bacteria in the wash solution could potentially spread to other originally un-contaminated produce and aggravate the food safety issues. In this study the authors determined the levels of chlorine concentration in wash water necessary to prevent survival and spread of bacteria from contaminated to uncontaminated lettuce when freshly cut lettuce is added to the wash water. Properties that influence the wash water quality were measured before and after washing. This information is important to the produce industry in designing wash operations in order to maintain effective chlorine level in preventing bacterial survival and spread during fresh-cut produce washing.

Technical Abstract:
This study investigated the effect of free chlorine level in wash water on Escherichia coli O157:H7 reduction, survival and transference during fresh-cut lettuce wash, and determined the minimum free chlorine level required to prevent pathogen cross-contamination. The effectiveness of rewashing for inactivation of E. coli O157:H7 on newly cross-contaminated produce previously washed with insufficient chlorine was also evaluated. Results indicate that solutions containing a minimum of 0.5 µg/ml free chlorine were effective to inactivate E.coli O157:H7 in suspension to below the detection level of 0.12 MPN/ml. However, the presence of 1 µg/ml free chlorine in the wash solution was insufficient to prevent E. coli O157:H7 survival since the introduction of cut lettuce to the wash system quickly depletes free chlorine. Although no E. coli O157:H7 was detected in the wash solution containing 5 µg/ml free chlorine prior to washing a mix of inoculated and un-inoculated lettuce, low numbers of E.coli O157:H7 was detected on un-inoculated lettuce on four of the seven experimental trials; yet, no E.coli O157:H7 was detected on the un-inoculated lettuce when free chlorine level was 10 µg/ml or greater. This suggests that the presence of a 10 µg/ml free chlorine prior to wash was sufficient to prevent pathogen cross-contamination. Re-washing newly cross-contaminated lettuce in 50 µg/ml free chlo rine for 30 sec significantly reduced the E.coli O157:H7 populations; but failed to eliminate E. coli O157:H7 cells.